Date of Award

Spring 1997

Document Type

Thesis - Restricted

Degree Name

Master of Science (MS)

Department

Civil and Environmental Engineering

First Advisor

Zanoni, Alphonse E.

Second Advisor

Crandall, Clifford J.

Abstract

Protecting residential plumbing systems from scale-forming precipitate deposits is most commonly accomplished by ion-exchange water softening equipment. However, these systems require disposal of waste brine solutions. A solution to this disposal problem might be to employ other softening methods currently used by large scale municipal water treatment plants and modify them to suit residential use. Several treatment methods are available to remove hardness constituents from water. These hardness constituents are primarily calcium or magnesium salts of sulfate or bicarbonate. Hardness removal or water softening has traditionally been performed by ion-exchange, precipitation by chemical addition, or demineralization by reverse osmosis. Lime softening precipitation is widely used, but requires large quantities of lime which lead to sludge disposal problems. Ion exchange is used where the cost of the ion exchange resin and regeneration of the resin are not prohibitive. Electrodialysis has been used, but it too can be relatively costly. Reverse osmosis membranes have also been employed for water softening. However, reverse osmosis membranes in general have required relatively high operating pressures and have been viewed as energy intensive and expensive to maintain. Nanofiltration membranes offer a modified version of reverse osmosis membranes that are able to remove a significant portion of the hardness cations and yet can be operated at much lower pressures than reverse osmosis membranes. The lower operating pressures result in less energy consumption and, therefore, lower operating costs. As with any type of membrane process there exists a potential for fouling of the membrane. Fouling is the resistance to flux (the permeation rate of water through membranes) by contaminants or foulants in the untreated water. These contaminants can either temporarily or permanently affect the membrane. Temporary reductions in flux can usually be reversed by cleaning the membrane. Fouling can be associated with either biological or physical contamination. Effective pretreatment systems can be used to eliminate or slow the fouling process and thus lengthen the life of the membrane and time between cleanings. Investigations have revealed that certain types of pretreatment can reduce or eliminate membrane fouling; however, the type of pretreatment often depends on the type and quality of raw water supplied to the system. Models exist to predict system behavior based on certain water quality parameters and operating values; however, these models are not always accurate and often over simplify or ignore some scaling parameters. For these reasons pilot scale studies are often needed to more accurately determine the be.st operating parameters and pretreatment systems required for the type and quality of raw water supplied to the system.

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